So Whatever Happened to the Flu?

(PhysOrg.com) -- Normally, this is the peak period for the flu in the United States. But that just doesn't seem to be the case this year.

The number of states reporting widespread cases of the flu mysteriously plunged from 49 at the end of October to zero at the beginning of the January, according to the Centers for Disease Control and Prevention. At the beginning of March, most states are only reporting sporadic cases of the flu.

It seems only a few short weeks ago when medical authorities declared a flu pandemic, the first in more than 40 years, and warned that because of the simultaneous existence of both the H1N1 and regular seasonal flu strains, the 2009-10 flu season could wind up as one of the worst on record.

So what happened?

Only time will tell, said Dr. Michael Koller, associate professor, in Loyola University Chicago Stritch School of Medicine, Maywood. Flu pandemics are highly unpredictable and no one can say for certain when one will start, when it will end and how severe it may be. A particular flu strain can go "poof" and disappear, or it could mutate and ramp up with a deadly vengeance, as has happened in the past.

"This flu season isn’t like any other I've ever lived through," Koller said. "We haven't had a flu pandemic since 1968 and all the rules go out the window with a pandemic. We don't know what to expect."

In a typical year, the regular flu season begins in November and reaches its peak in February, Koller said, although cases can occur as late as May. Older people, young children, pregnant women and people with chronic health conditions such as respiratory illnesses and heart disease are at an increased risk for serious complications from the regular flu strain.

This flu season, however, began far earlier, in April, with the appearance of H1N1, also known as "swine flu," and continued to cause illness throughout the summer, which normally doesn't happen with the regular seasonal flu, Koller said. In addition, illness from the H1N1 strain tended to be more severe than the seasonal flu among people younger than 65.

Though we might have dodged the worst this year's flu season, caution is still in order, Koller said. Infections are still expected to occur for weeks, possibly resulting in another wave of widespread illness.

"The Spanish flu pandemic came in three waves with the first occurring in the spring of 1918 and the third hitting a year later in March 1919," Koller said. "We wouldn't be surprised if we have another wave with this pandemic."

Each year in the U.S. between 5 to 20 percent of the population contracts the seasonal flu. Most people recover in a few days but in extreme cases, the flu can lead to pneumonia or death. About 36,000 Americans die and 200,000 are hospitalized from the flu each year.

As far as H1N1, the CDC estimates that as many as 84 million infections occurred in the United States between April 2009 and Jan. 16, 2010. During that period, up to 378,000 people were hospitalized due to complications from H1N1 and 17,160 deaths occurred.

As always, the best protection against the flu is immunization, Koller said. Fortunately, this year’s vaccine has been a good match to the viral strain circulating in the U.S. So getting a vaccine this late in the season can still offer protection.

"Now is a good time to get vaccinated because it takes about two weeks to develop an antibody response after the flu shot," Koller said.

Also, it's a good idea to continue to practice good personal hygiene such as covering your mouth when you cough and covering your nose when you sneeze and it's really important to wash your hands, Koller said.

"Flu is primarily spread by respiratory droplets," Koller said. "When somebody with influenza coughs or sneezes, out shoots this spray of flu virus that can infect anyone nearby."